For a conventional movable device, an error occurs after moving through a given distance in either linear movement or nonlinear movement, because of environmental factors (such as variation of distance caused by thermal expansion), human factors (such as poor quality control in production process and variation existing between movable devices of different production batches or different manufacturers), and other factors. The longer the distance that a movement takes is, the accumulation of error will be greater, making it not suitable for precision machining operations. Consequently, the moving distance shown on a controller for a movement taken by the movable device is different from a true distance detected through precision measurement (such as an optic scale) simply due to such errors. Today, robots (or manipulators) have been used to replace manual operation in order to cut down cost and eliminate errors and thus improving manufacturing efficiency. The preciseness of such devices must be set in a severer standard.
As to reduction or error, the conventional ways are periodic inspections (such as daily inspection, monthly inspection, or season inspection) and calibrations. These ways do not allow instantaneous correction. Once an inspection of a finished product indicates that the allowable error tolerance is exceeded, repair of rework must be performed. This increases the manufacture cost, not to mention that such a product may be totally scrapped due to repair being impossible. In addition, the process of shutting down system for calibration is simply wastes of labor and time and affects the production.
Thus, the present invention aims to provide a movable device that allows instantaneous correction of error in a moving situation in order to prevent accumulation of error, so that the error can be controlled within an allowable range regardless how long the moving distance is.